1. Field of the Invention
This invention relates generally to practical display units for electronic directories and like electronic tabulations for public viewing; and more particularly to a display unit for an electronically controlled directory that employs a liquid-crystal display.
2. Prior Art
Directories are commonly posted in the public lobbies of business buildings, apartment houses, multiple-building condominium complexes, and other multiple-occupant facilities. In secured facilities, the entries in such directories often include room or suite numbers, or other numbers for use with an adjacent telephone or intercom in contacting individual occupants to gain admission.
In a few large facilities, recently, hand-lettered or movable-letter directories have given way to electronic systems that are much easier to revise. Such systems eliminate tedious manual reshuffling of placards or letters to keep entries in alphabetical order and to accommodate subdivision or consolidation of occupant suites.
Although they are an enormous improvement over manual directories, the electronic systems have suffered from a major limitation in their use of cathode-ray-tube (CRT) display units. Such video display units, in the forms currently encountered in commercial practice, ave several well-known drawbacks.
The drawbacks of CRT displays include image instability, poor resolution and (particularly in bright light) poor contrast. Instability of the image, ranging from minor flicker to vertical roll, can make reading the information on the screen difficult.
Poor resolution severely limits the number of entries that can be displayed simultaneously on a screen of moderate size. This strategy sometimes leads to very large screens that visually dominate a lobby.
Some system designers attempt to avoid this drawback by programming the units in operating modes that call upon a visitor to "page through" different screens to find a particular occupant. The "page through" mode itself is in principle entirely acceptable, but when the number of entries on a screen is unduly small--so that a typical visitor must search through several screens even for a relatively small directory--the typical visitor justifiably becomes annoyed. That is what happens with a CRT display, because of its limitations.
Alternative methods for locating an occupant without paging are either more complicated or more expensive, or both. For example, some systems provide a large alphanumeric keypad and require the user to spell the first few letters of the occupant's name. This increases the system cost and also reduces convenience, particularly for a user who is unsure how to spell the name.
In general all these drawbacks also detract from efforts by facility management to establish an elegant or prestigious style in a lobby or outdoor entry area. Poor contrast is considered among the worst offenders in this regard.
Sometimes, in the interest of offsetting poor contrast, directory system designers introduce the use of colors in the video display. Often, however, this strategy is counterproductive, because the CRT or video colors inject an incongruously gaudy element into a fine decor.
CRT displays are particularly troublesome in brightly lit environments such as outdoors and in lobbies surrounded by large windows that admit brilliant sunlight. In these circumstances, contrast can be so inadequate that the displays are almost completely unreadable.
Moreover, CRT displays are relatively expensive. In large formats they are too deep (front to back) for straightforward mounting in a wall--and so require provision of a free-standing or recessed support structure enclosure two or three feet deep. Because of their evacuated-chamber construction, they are also relatively fragile and inordinately subject to vandalism.
Other types of display are known for use with electronic information processors, but heretofore not with electronic directories or other large electronic tabulations for public viewing. Such other types include the light-emitting diode (LED) and the liquid-crystal display (LCD).
Most LEDs require relatively bulky apparatus for each character to be displayed. Furthermore LEDs are quite dim, and in the few very-small-screen outdoor applications where they have been used (such as some automatic-teller machines) they are extremely hard to read--even when elaborately shaded. A larger LED array such as required for a directory would be prohibitively difficult to shade effectively and would be inordinately expensive.
Under ideal conditions, liquid-crystal displays are capable of excellent contrast and resolution, are plainly readable even in the brightest light, and are readily backlighted for nighttime use. Their use would also result package. LCDs would accordingly be excellent candidates for directories and the like, but we are not aware of any prior suggestion for such use; and they do have important limitations.
An LCD has a display medium--the liquid-crystal fluid itself--and a structure which contains the fluid. In at least some commercial LCDs this structure typically includes two planar pieces of material with the medium sandwiched between them. At least the piece on the viewing side of the sandwich, which in this document we will call the LCD "face," ordinarily is transparent glass or plastic.
Electrodes are formed on the opposed interior surfaces of this sandwich. These electrodes too are ordinarily transparent on at least the face side. One electrode material is intrinsic tin oxide.
Both the fluid and the glass are very sensitive to temperature. (It may be recalled that the early applications of liquid-crystal displays were as novelty items, particularly including thermometers.)
If the temperature of the glass rises beyond certain relatively narrow limits, the display develops dark spots, or the entire display may actually turn dark. As we understand it, this darkening is due to an expansion of the cell gap within the glass. The black characters or other symbols then fail to stand out well against the darkening background.
On the other hand, if the temperature falls too much, the changing of characters begins to be very slow, an effect which is said to be related to increasing viscosity of the medium. As temperature decreases further the display blushes a different color (e.g., pink)--this time because of the cell gap's contraction--and again becomes unreadable.
Directory applications would call for use of the larger graphic LCDs, and also for a type of medium known as "supertwist" fluid. This kind of fluid provides far superior contrast and hence significantly better readability. The large LCDs, however, and especially those using supertwist fluid, are particularly sensitive to temperature.
Presumably for these reasons LCDs heretofore have been used primarily in applications involving small formats or intrinsic temperature control, or both. Thus LCDs are employed extensively for wristwatches--since they can make good use of LCDs that are smaller and nonsupertwist, and therefore less temperature-sensitive. LCD wristwatches also take advantage of the wearer's limited temperature tolerance, and heat conduction to and from the wearer's body, to limit the severity of temperatures to which the display is exposed.
Even under such relatively protected conditions, fading and blushing or wristwatch displays is well known to athletes and workers whose activities reach the anticipated design limits of the watches.
LCDs are also used for many usually indoor applications such as calculators and laptop computers. Here too they are typically used in temperature-controlled environments, or if they are found to malfunction can generally be moved into such environments.
Operation of large LCDs would be subject to temperature problems in lobbies and other indoor entryways, as well as outdoors, if the locations receive intense sunlight. Temperature rise in such areas sometimes outstrips the capabilities of a building air-conditioning system, and can be severe enough to degrade the performance of an LCD.
If the LCD were inside a case, and were protected from vandals by an unbreakable window, as is desirable in our application, the temperature problem would be aggravated much further. This would be due to a "greenhouse" effect, in which stagnant air between the window and the LCD becomes extremely hot, much like the interior of a car left shut on a hot day.
A related problem of LCD temperature sensitivity involves a voltage that is applied to the display medium to control the contrast of the characters relative to the background screen. The necessary voltage for proper contrast varies very strongly and nonlinearly with temperature.
Thus, as the temperature to which the LCD is exposed changes (e.g., between day and night), the LCD contrast would have to be constantly adjusted to prevent its characters from disappearing or becoming illegible. A very nonlinear relationship between the voltage and the temperature renders the problem of automatic contrast-control technique far from straightforward.
For whatever reason, LCDs have not been used in sizable directory-type displays. We will return to more general discussion of electronic directories and the like.
Another problem arises in configuration of such directories and the like, when outdoor or bright-lobby applications are involved. That is the problem of controlling reflections at glass or plastic surfaces of the display unit.
Such reflections of the viewing person--and of objects around and behind that person, become confused with the displayed characters, making them very hard to read. When sufficiently bright, these reflections actually obscure the displayed characters.
As a verbal shorthand we shall refer to these confusing and obscuring reflections collectively as "glare." Such glare can be controlled to a certain extent by providing a matte finish on the screen itself, provided that the electronic display screen (such as a CRT screen) is directly exposed to the viewing person. Direct exposure of the display screen is accordingly a conventional teaching of the prior art.
This conventional teaching, however, severely limits the use of electronic directories since it makes them susceptible to vandalism. Direct exposure of the display screen makes it easy for a vandal to break the screen or damage the display unit.
Hence there is a conflict between the direct exposure desired to control glare, and the interposition of an intermediate protective window desired to control vandals. This conflict is present with CRTs and LEDs--but particularly acute with LCDs because of the very way they work.
CRTs and LEDs inherently generate their own illumination, but LCDs usually depend upon incident light for their characters to be seen. Different portions of the fluid will either absorb or reflect incident light, and thus form visible characters, depending on whether or not the fluid is electrically excited.
This mechanism explains why LCD characters do not seem to fade in direct sunlight as do CRT and LED characters. Light that is reflected at a glass or plastic surface, however, is light not used to develop visible LCD characters--and, in fact, is light that creates reflections which compete with the already diminished LCD characters.
Thus, again, even if there had been a suggestion of LCD use in electronic directories, such a suggestion would have been particularly likely to meet with immediate rejection in view of the relatively adverse glare-related properties of LCDs.
Finally, even though the electronic directory systems currently available are far more convenient in terms of entering and deleting names than the movable-letter or placard directories, they still require local procedures for entries or revisions--either at the system itself or through a computer close by. This arrangement is very inconvenient for buildings that have off-site property management, especially when tenant turnover is high and frequent directory changes required.
All of the above limitations have resulted in the relatively limited use of currently available electronic directories. As can now be seen, the prior art has failed to provide an adequate display system for directories and the like, particularly for use out of doors and in lobbies or other entryways subject tointense sunlight.